Fix assigner service announce endpoint

[gammazero]

Need assigner to use newer /announce HTTP endpoint to be compatible with go-libipni HTTP sender.

Fixes ipni/go-libipni#36

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.25 🎉

Comparison is base (d08c11f) 51.24% compared to head (838d113) 51.49%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1754      +/-   ##
==========================================
+ Coverage   51.24%   51.49%   +0.25%     
==========================================
  Files         101      101              
  Lines       11250    11253       +3     
==========================================
+ Hits         5765     5795      +30     
+ Misses       4901     4872      -29     
- Partials      584      586       +2     

Impacted Files	Coverage Δ
assigner/server/server.go	41.58% <100.00%> (+1.78%)	⬆️

... and 1 file with indirect coverage changes

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